The present invention relates to a controller for fluorescent tubes wherein the tube is energized at the zero phase of the current, voltage or power cycle and wherein the tube is dimmed by turning off the tube at a phase angle selected to produce the desired amount of dimming. More particularly, the present invention relates to such a system wherein the counter electromotive switching spikes generated as a result of phase turn off of a reactive type load such as that found with respect to fluorescent tubes are converted into power pulses which are utilized during switch off intervals to achieve corrective work functions such as maintaining the filaments of a fluorescent tube heated during the switch off intervals. The present invention is particularly useful in the control of rapid start ballast fluorescent lighting systems.
Heretofore, most controllers of inductive fluorescent lighting loads have energized these loads at a variable phase angle turn on point and allowed natural commutation, i.e. the zero crossover point in the current and/or voltage cycle, to switch off the load. Other controllers have turned the fluorescent lighting loads on at the zero crossover point and have turned the loads off at a selected phase angle to yield the desired amount of lighting control.
Phase angle turn on controllers, i.e. controllers of the first type described above, which power rapid start ballast fluorescent lighting systems have a very limited control range because the regulatory effect of the choke-capacitor components in the ballast tends to counteract any change in RMS load voltage and because there is a rapid drop in fluorescent tube heater voltages as the angle of turn on is increased.
Phase angle turn off controllers, i.e. controllers of the second type, produce a stronger counter electromotive force spike from the rapid start ballast inductance. This spike causes severe acoustic noise and break down of circuit components. Prior art devices have heretofore suppressed this spike with some loss of power. However, rapid start ballast circuits connected to a turn off type of controller employing spike suppression experience low fluorescent tube heater voltages at lower dimming levels resulting in a reduced dimming range.
It has not been possible prior to the present invention to achieve effective and reliable dimming control of rapid start fluorescent ballast lighting systems because the reduced operating voltages imposed upon the fluorescent tube electrodes at lower light levels causes poor tube ignition, causes premature tube drop out, and lessens tube life due to cathode stripping. The regulatory effect offered by the series connected choke and capacitor arrangement in the rapid start ballast opposes attempts to control or modulate the ballast AC input. Extremely high amplitude counter electromotive forces or flyback spikes resulting from the turn off control of rapid start ballast systems causes unacceptable ballast acoustic noise levels, causes poor fluorescent tube crest factors, and endangers circuit components. A further disadvantage of existing control methods is that low light levels are susceptible to light intensity changes caused by line voltage changes.
The present invention overcomes many of these difficulties by utilizing a half wave turn off method wherein the series inductive capacitive regulatory characteristic of the rapid start ballast is overcome. The present invention provides for reshaping and reenforcing of the counter electromotive force wave during the switch off cycle thus producing a much improved quasi-sinusoidal wave power pulse which is used to maintain fluorescent tube heater voltages, provide improved tube ignition, lessen premature tube conduction drop out, improve fluorescent tube life, extend dimming control range, and improve tube crest factors.